The role of HT1 protein kinase in red light-induced stomatal opening
Abstract: Red light and blue light induce stomatal opening in plants via separate pathways. It is unclear to what extent photosynthetic processes, if any, of guard cells or underlying mesophyll are involved in the red light-induced stomatal response. The HT1 protein kinase is a negative regulator of high CO2-induced stomatal closure and the ht1 mutant responds to blue light and ABA. If the red light signal is transduced via a change in photosynthetic activity, this mediator signal could be an altered intercellular [CO2]. Due to the stomatal CO2 insensitivity in ht1, this mutant was analyzed for its role in red light-induced stomatal opening.Gas exchange measurements of stomatal conductance showed a lack of functional red light stomatal opening in ht1-1 and ht1-2, while ht1 alleles exhibited stomatal opening under blue light. In order to examine photosynthesis processes, measurements of photosynthetic assimilation, several fluorescent parameters and a net carbon assimilation assay were performed. The results show that mesophyll photosynthesis is largely functional in ht1-2 both for light-dependent and Calvin Cycle-dependent reactions. Despite red light illumination, fusicoccin induced H+-ATPase activity and stomatal opening in ht1 mutant as examined by microscopy-based bioassays. To assay whether other red-light triggered processes are functional in ht1 mutants, the de-etiolation response was investigated. The wild-type response of ht1 suggested that an impaired phytochrome B performance is not the cause for an impaired red light stomatal response. In addition, a method to perform guard cell protoplasts isolation was established for use in future cell-specific applications. Putting the findings in perspective led to a model in which HT1 functions within guard cells as a positive mediator of red light and low CO2 control of stomatal opening, where HT1 acts prior to H+-ATPase activation.
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